The present invention relates to process control systems and, more particularly, to a microcomputer system for controlling a process by performing arithmetic operations with a microcomputer.
In process control systems, a computer process control system with a microcomputer therein has been developed to control the process without the necessity of the conventional instrumentation. The process control system with a microcomputer can freely perform various control operations such as feedback control, cascade control, feed forward control, sampling control, sequence control and so on in accordance with corresponding software. The free combination of control operations provides a variety of process applications and highly precise control characteristics.
A single-loop controller includes a man-machine interface and a microcomputer (control device) to perform direct digital control (DDC) which responds to a trend toward diversification of the control system and which is directly coupled to the process to control it. The man-machine interface and the microcomputer are housed together in a single housing.
Analog detection signals are transmitted from a plurality of detection terminals or detectors which detect predetermined kinds of variables relating to physical or chemical processes and the like and are selected by a multiplexer. The selected analog detection signal is then converted to a digital signal. The digital signal is supplied to the central processing unit (CPU) of a microprocessor. In the CPU, the digital signal is processed in accordance with a preset operation program. The operation result is stored in an up/down counter. A preset value (target value) corresponding to the reference operating condition of the process detected at the detector is preset in the up/down counter by the CPU. If a disturbance (a source which interferes with automatic control) occurs in the process, the CPU compensates for the disturbance by generating a command signal so as to match a variable of the process and the target value. The command value indicated by the command signal is temporarily digitally held in the up/down counter and is then converted to an analog signal. The analog signal is further converted to an analog operation signal which is supplied to an operation terminal or actuator. As a result, the process is automatically controlled so that the variable thereof reaches the preset target value.
It is very useful for a DDC operation to use the conventional single-loop controller in which the man-machine interface and the control device are integrally assembled. However, if the single-loop controller is applied to a large-scale instrumentation system, especially, if a total display device such as a cathode-ray tube (CRT) is assembled in the system to perform centralized control of the process and to achieve a so-called panelless operation with a CRT display, its usefulness is degraded. In other words, if a plurality of single-loop controllers each one of which has a non-separable man-machine interface corresponding to analog instrumentation are arranged, similar man-machine interfaces are continuously aligned, and the total area of the panels is undesirably increased, resulting in waste of the space of the instrument room. Further, if the CRT display device is used, man-machine interfaces corresponding to the number of instrumentation loops need not be used. Therefore, the man-machine interfaces of the CRT overlap the CRTs, and/or the transmission systems also overlap each other. Further, it is inefficient for the operator to supervise all man-machine interfaces including the man-machine interfaces which overlap the CRTs.
Since the automatic control equipment for computer processing is used in a variety of applications, it is strongly desirable that the equipment, including the CRT display device, be freely designed and assembled at an instrumentation engineering level and that the control functions also be improved. The conventional single-loop controller fails to satisfy the needs for easy and various applications in large-scale instrumentation as well as small-scale instrumentation.
In order to solve the above problem and to satisfy the needs described above, a man-machine interface and a control device of a single-loop controller are separated and assembled in an instrumentation system in accordance with each level after separation. However, even if the man-machine interface and the control device are simply separated and assembled in separate housings or cases, respectively, and are connected by cables, the above arrangement does not provide an effective solution because wirings are complicated. If the control devices are housed in a cabinet, cables from the process and separated man-machine interfaces are concentrated at one part of the cabinet. The number of cables is increased if the number of loops is increased, resulting in disadvantages from the viewpoint of inspection and maintenance. Further, room for the additional cables to allow system expansion is so small that the system cannot be effectively expanded.
Further, according to the above arrangement, when a broken control device is disconnected from the system, little or no operation output is produced from the CPU, and the process control may be greatly damaged. During the period for which the broken control device is removed from the system, it is difficult to back up the system by manual control with the man-machine interface.
The effective solution cannot be presented simply by separating the man-machine interface from the control device of the single-loop controller.